Valved paper sacks are used extensively for packaging pulverulent or granular material. The scatterable or pourable material can be introduced on automatic filling equipment, and the valve, which self-seals, after filling, through the pressure exerted by the contents, in most cases makes an additional process step for sealing the filled sack unnecessary; alternatively, additionally applied sealing sheets are subjected to substantially less stress because of the self-sealing properties of the valve.
In addition to the extensively employed stitched valved sack, the block-bottom valved sack is very important. Whilst the stitched sack on the one hand has the advantage that it is inexpensive to manufacture, it also reveals the disadvantage of not being absolutely tight. On the other hand, a block-bottom valved sack exhibits good tightness but against this is more expensive, and requires more material, in its manufacture.
European Patent Application No. 82/108,232.8 has proposed a gusseted valved sack in which the valve is introduced into the lengthwise weld of the piece of tubing, in the vicinity of one of the two folded bottoms of the sack. This gusseted valved sack is less expensive to manufacture than a block-bottom valved sack; moreover, it is even superior to the latter in tightness.